American Johnson Wagner had his lead at the Shell Houston Open cut to a single shot as Chad Campbell and Bob Estes closed in.

Wagner - who led by three at the halfway stage - shot a three-under-par 69 to move to 15 under par.

His fellow American Campbell had a seven-under 65 to move to within a shot while Estes is two further back after a sizzling 64, alongside Charley Hoffman.

Wagner started with three pars before a birdie on the fourth. He then had a bogey five on the fifth when he missed a six-foot par putt.

"I was definitely nervous starting the day," he said.

"I had some good chances early. But putts weren't really falling, and I made a bad bogey on five."

Wagner responded with three birdies in four holes from the ninth through to the 12th.

"I really held it together, hit a bunch of greens - and didn't really have to work too hard," he added.

Estes, who began the round eight strokes behind, stormed up the leaderboard with a front-nine 31. Three more birdies then followed from the 11th to the 13th.

Campbell caught Wagner at 12 under when he holed out with a wedge from 88 yards - and then again at 13, 14 and 15 under - until he made a bogey on the 17th, failing to get up and down from a greenside bunker.

Fred Couples, who was in a tie for 36th, joined a group of six others for 10th by signing for a 67.

World number two Phil Mickelson shot a one under 71 to lie 22nd.

With no realistic chance to win, Mickelson is already looking to next week and The Masters at Augusta.

"I'm going to try to get a good solid round and see if I can make some birdies and get momentum for next week's Masters," the two-time Augusta champion said.